Critical Ceiling Treatment for Immersive Audio

[VinceHoffman]

My 10'x4'x5" air-gapped ceiling cloud does double duty. It intercepts the primary ceiling reflection for both the LCR mains and front Atmos overheads.

 

[hemiutut]

My 10'x4'x5" air-gapped ceiling cloud does double duty. It intercepts the primary ceiling reflection for both the LCR mains and front Atmos overheads.

View attachment 46016

Do you have measurements made with the Room EQ Wizard without and with the acoustic cloud?

Written with translator.

Greetings

 

[VinceHoffman]

No I currently do not. At the time nearly a decade ago, a local REW wiz and acoustic consultant did help me perform baseline measurements of my room, system, and prior DIY treatments along with several during and after measurements of the upgraded broadband DIY treatment strategy. Unfortunately those REW measurement files were long ago lost in a stolen laptop that wasn't backed up properly.

My prior DIY broadband absorptive treatments seen in the collage below were all several inches shallower than my current panels. The earlier panels and 8'x12"x3" DIY "flag style" ceiling mounted absorbers only featured 2" of rigid fiberglass, and therefore were not as broadband as they ought to have been. While not perfect they were all a markedly audible improvement over bare walls and ceiling when originally installed in my then new dedicated room 20+ years ago.

However a decade or so later, the recommended replacement 5" deep (4" of OC703 rigid fiberglass + airgap) DIY panels seen in the the pic below along with the install of that new larger ceiling treatment and poly diffusers up higher on the walls, offered sonically and measurably significant improvements over my prior room treatments. The moral of the story is that shallow broadband treatments simply aren't sufficiently broadband to gobble up the entirety (250Hz-20kHz) of unwanted aurally confusing primary reflections. Besides improving the RT60 waterfall measurements in the octaves between 250Hz and 1kHz, the uniform wider band absorption resulted in comparatively more open sounding high frequencies.

 

[hemiutut]

Think the same, absorbent panels 5 "thick are not valid, since they will absorb little from 400 Hz + or - up to the Schroeder frequency of our rooms.
From there down a multisub configuration and it will not be necessary to put any resonator.

I see many rooms with very narrow panels and on top of that,if they are batcave with carpets,the worst is to offset the reverberation.

Written with translator.

Greetings

 

[VinceHoffman]

Think the same, absorbent panels 5 "thick are not valid, since they will absorb little from 400 Hz + or - up to the Schroeder frequency of our rooms.
From there down a multisub configuration and it will not be necessary to put any resonator.

I see many rooms with very narrow panels and on top of that,if they are batcave with carpets,the worst is to offset the reverberation.

Written with translator.

Greetings

I don't use these panels as bass traps and I don't disingenuously describe them as such. A true velocity based bass trap must be many multiples of this depth in order to usefully absorb excess modal frequency bass misbehavior. In my room I use 13", 16", and 20" diameter ASC Tube Traps to address modal frequency & SBIR issues. However a 5" deep panel is perfect for primary reflection duty along sidewalls or ceiling.

5" depth broadband panels used along the sidewalls or ceiling at primary reflection points are intercepting sound arriving at the panel face at an oblique angle. If you perform the geometry calculation, a reflection arriving at a 45 degree angle to the panel face travels 1.4x the panel depth while passing through it. So for a 5" panel depth, we arrive at the incident sound wave traveling 7" on its way into the panel, reflecting off the boundary behind, and then traveling through an additional 7" of fiber on it's way back out. 14" is the 1/4 wavelength of 241Hz and the panel will efficiently absorb all frequencies above the 1/4 wave depth.

So while not a bass trap, a 5" panel is sufficiently broadband to completely absorb the spectral content of primary reflections between 250Hz and the limits of human hearing. We entirely eliminate the unwanted early reflection that would otherwise cause image and soundstage confusion for the ear-brain. Also for this use case, a 5" panel is sufficiently broadband to avoid tipping the overall combined direct & reflected sound spectral balance towards sounding dull or dead.

 

[hemiutut]

In my case I am clear about what material to put and its thickness and below the Schroeder frequency of each room, the best thing is a multisub configuration without putting any resonator.

In this link you can see a book waterfall with 4 subwoofers.

Even playing with equalization already improves the waterfall in low frequencies.

https://gearspace.com/board/studio-...ng-even-out-freq-response-9.html#post12491247

http://gikacoustics.co.uk/wp-content/uploads/2013/11/RAL-Report-Full-Range-Monster-Bass-Trap.pdf

One example are these 7.5" thick panels.
In the test they used 8 panels as commented.

My panels start at 8 '' thick and I use Dacron for filler material.
Located at the maximum speed for the best optimization.


Greetings

 

[VinceHoffman]

In my case I am clear about what material to put and its thickness and below the Schroeder frequency of each room, the best thing is a multisub configuration without putting any resonator.

In this link you can see a book waterfall with 4 subwoofers.

Even playing with equalization already improves the waterfall in low frequencies.

https://gearspace.com/board/studio-...ng-even-out-freq-response-9.html#post12491247


View attachment 46063View attachment 46064
View attachment 46065View attachment 46066

http://gikacoustics.co.uk/wp-content/uploads/2013/11/RAL-Report-Full-Range-Monster-Bass-Trap.pdf

One example are these 7.5" thick panels.
In the test they used 8 panels as commented.

My panels start at 8 '' thick and I use Dacron for filler material.
Located at the maximum speed for the best optimization.


Greetings

In the example of SteveGTR on Gearslutz/space which you provided, Steve used significant DSP EQ on both the mains and the subs to achieve this result. With an all digital signal path in his studio environment this is a reasonable approach. The worst of the response aberations seen in the before pic are actually all above the subwoofers' passband. The 60Hz resonance seen in the before pic is an electrical ground loop buzz (not an acoustic problem) which they had obviously resolved by the time the final measurements were taken. Frankly the subwoofer region seen in the before pic looks mighty smooth to begin with.

I too use four subwoofers in a Toole/Welti inspired distributed base arrangement. This approach definitely averages out the worst of the modal issues in the subwoofer passband. However it does not create bass perfection on its own. And DSP based EQ is a powerful tool but in my experience there are sonic downsides to applying it full range. That goes doubly so for analog sources which then must be digitized prior to applying DSP EQ. I only apply DSP EQ and time alignment to the subwoofer array. For two channel replay, I use a sonically transparent analog crossover (JL Audio CR-1) to maintain an analog signalpath above the 80Hz crossover point where the subwoofers hand off to the same ATC SCM20ASL monitors SteveGTR on GearSpace used. Passive floor-to-ceiling bass trapping in the corners SBIR bass trapping along the sidewalls looks after much of the mid and upper bass issues without resorting to full range DSP.

 

[hemiutut]

That's right, it uses an 8-filter DSP, which is clear that the less treatment there is in the room, the more equalization will have to be used and yes, equalization will always have to be used although that is not liked by audiophiles.

In this room it is perfectly seen as there is EQ and not little, despite having acoustic treatment.

It is best to read the comments of the owner of the room talking about the equalization applied in his equipment

At first I wasn't thrilled with the results. I believed the transients were rounded at the edges far too much for my taste. This coupled with the fact that I was used to listening without any DSP for my entire life, made me frustrated. I reported back to Mitch everything I heard, liked, and disliked. His response to me was very reassuring. He said not to worry because there are many industry standard curves to try and many small adjustments he can make to the filter.

Over several weeks Mitch sent me more filters to try. I was traveling much of this time, so I assume we could've hammered out my issues pretty quick if I could dedicate more time to the effort. Mitch sent filters based on ITU, B&K and Bob Katz target curves before sending the EBU 3276 target curve. I liked the direction he was taking my system with the ITU curve, but when I pressed play with the EBU 3276 curve enabled, I was sold. My system was really singing. My room was out of the picture, and the music was right there in front of me.


https://audiophilestyle.com/ca/bits...-part-two-acoustics-speakers-dsp-r863/page/2/

My in-room response before (top) and after (bottom) DSP / room correction using the EBU 3276 target curve.

Greetings

 

[welldun]

Good day all, thanks for the thread. I have a question regarding the ceiling treatment ( and wall treatment) for a case like mine in which I actively use ceiling reflections for Atmos enabled speakers, but also have the center channel mounted above the projector screen, which is probably creating unwanted reflections somewhere off the ceiling. This room is somewhat barn shaped so only the central part (front to back) of the ceiling is flat as in parallel to the floor and left and right of that flat part the ceiling angles down towards the walls. So I'm wondering how I can best go about finding the exactly reflection points for the mains and center (LCR) that should be treated as well as the areas of the ceiling that should be left alone so as to not diminish the effect of the atmos enabled speakers? I'm pretty good at taking measurements with REW, just need help interpreting those measurements to find the trouble areas that if addressed could make an audible difference not just one that looks better on a graph. The room is roughly 18feet x 13feet x 7.5 feet. with a 7.4.4 speaker setup.

 

[DanDan]

4" of OC703 absorbs 84% in the 125Hz Octave Band, Thermafiber and others these days do even better. .

 

[welldun]

Hello Dan, thanks for the response and sorry for my late reply. not sure I didn't see your reply sooner.

For Dan and anyone else reading this, I've attached a picture of the ETC (with 0.2 smoothing) for my Center Channel speaker which is placed above the screen. I'm hoping someone can help me better understand the graph and how to figure out which of the spikes need to be addressed with absorption. From what I've read thus far, it seems like any spike that is taller than the preceding one is a reflection. Also, that the time in ms describes how much longer that reflection took to arrive to the measuring microphone. In my case, the first spike just after the direct sound seems to come in around 567u according to REW. If I'm understanding this correctly, that means that if I multiple this 567u by 1.126 (the distance sound travels in 1ms) then that first reflection is roughly 0.638 of a foot from the speaker?? If so, then this is roughly how close the ceiling is from that speaker. If that's the case, how do I calculate how much absorption I would use there, as well as the other spikes? Lastly, how after out in ms are we dealing with these spikes (at what point do we stop worrying about them?)
I realize that all of the above could be totally incorrect, so I humbly ask that anyone will to clear this up please chime in. Thanks.

P.s. I have plenty of measurements done in REW so if any others are needed, just ask.

 

[DanDan]

You are welcome. Happy New Year. That graph looks a bit odd. You have normalised it so the initial peak sound be at 0dBFS. The vertical range is way too big. For RT60 the maximum we need is 60dB, but few of our workplaces have ambient noise below 40dB so with 85dB nominal listening levels, a range of 0--40 is probably suitable. We generally focus on the first 20mS because our hearing blends that period together. Later we hear slaps and echoes. So, make sure you have just one speaker running, tweak your graph extents, mentally rename the ETC as EchoGraph..... In normal Critical Listening we want a clutter free 20mS at least. So no reflections louder than -20dBFS during the first 20mS. Modern CRs, even in the Prosumer scenario typically do much better than that, reaching pretty much Anechoic for this early period. I am bemused by Immersive, Dolby Atmos, Tidal MQA, Apple Spatiial...... We have had proprietary competing systems in the past. It does not go well, Betamax was Better!

 

[welldun]

Thanks again Dan. Let me know if this next picture is better? Also, was my explanation above correct with regards to how to use this ETC chart to find the reflection points?

 

[DanDan]

You are very welcome. You can see the reflections in excess of the 20/20 rule of thumb for Critical Listening. I am not fully up to speed for particular recommendations re Dolby Atmos or other protocols. But I do remember that an AES Surround group recommended a very different scenario. So maybe, no problem. But Early Reflections definitely cause conflict, comb filtering between the Direct to Ear, and Reflected paths. Those paths are the EXTRA or DELTA time it takes from Tweeter to Ear. A Pool or Snooker shot.